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| 20 | <head>
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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Local Labels (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Local Labels (GNU C Language Manual)">
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| 25 | <meta name="keywords" content="Local Labels (GNU C Language Manual)">
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| 29 | <link href="index.html" rel="start" title="Top">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Statements.html" rel="up" title="Statements">
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| 33 | <link href="Labels-as-Values.html" rel="next" title="Labels as Values">
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| 34 | <link href="goto-Statement.html" rel="prev" title="goto Statement">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Local-Labels"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Labels-as-Values.html" accesskey="n" rel="next">Labels as Values</a>, Previous: <a href="goto-Statement.html" accesskey="p" rel="prev">goto Statement</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Locally-Declared-Labels"></span><h3 class="section">19.13 Locally Declared Labels</h3>
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| 64 | <span id="index-local-labels"></span>
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| 65 | <span id="index-macros_002c-local-labels"></span>
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| 66 | <span id="index-_005f_005flabel_005f_005f"></span>
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| 67 |
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| 68 | <p>In GNU C you can declare <em>local labels</em> in any nested block
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| 69 | scope. A local label is used in a <code>goto</code> statement just like an
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| 70 | ordinary label, but you can only reference it within the block in
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| 71 | which it was declared.
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| 72 | </p>
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| 73 | <p>A local label declaration looks like this:
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| 74 | </p>
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| 75 | <div class="example">
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| 76 | <pre class="example">__label__ <var>label</var>;
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| 77 | </pre></div>
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| 78 |
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| 79 | <p>or
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| 80 | </p>
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| 81 | <div class="example">
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| 82 | <pre class="example">__label__ <var>label1</var>, <var>label2</var>, <span class="roman">…</span>;
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| 83 | </pre></div>
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| 84 |
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| 85 | <p>Local label declarations must come at the beginning of the block,
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| 86 | before any ordinary declarations or statements.
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| 87 | </p>
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| 88 | <p>The label declaration declares the label <em>name</em>, but does not define
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| 89 | the label itself. That’s done in the usual way, with
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| 90 | <code><var>label</var>:</code>, before one of the statements in the block.
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| 91 | </p>
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| 92 | <p>The local label feature is useful for complex macros. If a macro
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| 93 | contains nested loops, a <code>goto</code> can be useful for breaking out of
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| 94 | them. However, an ordinary label whose scope is the whole function
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| 95 | cannot be used: if the macro can be expanded several times in one
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| 96 | function, the label will be multiply defined in that function. A
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| 97 | local label avoids this problem. For example:
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| 98 | </p>
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| 99 | <div class="example">
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| 100 | <pre class="example">#define SEARCH(value, array, target) \
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| 101 | do { \
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| 102 | __label__ found; \
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| 103 | __auto_type _SEARCH_target = (target); \
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| 104 | __auto_type _SEARCH_array = (array); \
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| 105 | int i, j; \
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| 106 | int value; \
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| 107 | for (i = 0; i < max; i++) \
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| 108 | for (j = 0; j < max; j++) \
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| 109 | if (_SEARCH_array[i][j] == _SEARCH_target) \
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| 110 | { (value) = i; goto found; } \
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| 111 | (value) = -1; \
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| 112 | found:; \
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| 113 | } while (0)
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| 114 | </pre></div>
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| 115 |
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| 116 | <p>This could also be written using a statement expression
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| 117 | (see <a href="Statement-Exprs.html">Statement Exprs</a>):
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| 118 | </p>
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| 119 | <div class="example">
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| 120 | <pre class="example">#define SEARCH(array, target) \
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| 121 | ({ \
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| 122 | __label__ found; \
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| 123 | __auto_type _SEARCH_target = (target); \
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| 124 | __auto_type _SEARCH_array = (array); \
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| 125 | int i, j; \
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| 126 | int value; \
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| 127 | for (i = 0; i < max; i++) \
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| 128 | for (j = 0; j < max; j++) \
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| 129 | if (_SEARCH_array[i][j] == _SEARCH_target) \
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| 130 | { value = i; goto found; } \
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| 131 | value = -1; \
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| 132 | found: \
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| 133 | value; \
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| 134 | })
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| 135 | </pre></div>
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| 136 |
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| 137 | <p>Ordinary labels are visible throughout the function where they are
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| 138 | defined, and only in that function. However, explicitly declared
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| 139 | local labels of a block are visible in nested functions declared
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| 140 | within that block. See <a href="Nested-Functions.html">Nested Functions</a>, for details.
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| 141 | </p>
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| 142 | <p>See <a href="goto-Statement.html">goto Statement</a>.
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| 143 | </p>
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| 144 | <hr>
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| 145 | <div class="header">
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| 146 | <p>
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| 147 | Next: <a href="Labels-as-Values.html" accesskey="n" rel="next">Labels as Values</a>, Previous: <a href="goto-Statement.html" accesskey="p" rel="prev">goto Statement</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 148 | </div>
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| 149 |
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| 150 |
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| 151 |
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| 152 | </body>
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| 153 | </html>
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